Orthogonal Frequency Division Multiplexing (OFDM) has become a widespread technique in several wide-band digital communication systems. OFDM-techniques are used both in wire-bound systems such as ADSL and wireless systems such as WLAN. Recently OFDM has been selected as one of the transmission techniques in the next generation of mobile communication, 4G. OFDM offers high spectral efficiency and is fairly robust to interference and multipath propagation. Additionally OFDM can adapt to varying channel conditions without complex equalization.
In systems using OFDM, the symbols are. after channel coding and modulation mapped onto a two-dimensional time-frequency grid. This grid might also have a third dimension, the space dimension, if a wireless system utilizing multiple transmit antennas. The grid or matrix, which is a characteristic of the OFDM-techniques, defines resource-units which are the smallest addressable entities in the system. Illustrated in FIG. 1a is a two-dimensional OFDM grid with a vertical frequency axis, and a horizontal time axis. Four different frequencies, f1-f4, and seven time-slots. t1-t7. are utilized, each frequency/time-slot pair representing one resource-unit, giving 28 individually addressable resource-units. In FIG. 1b a space dimension is added, in the form of a 4 antenna scenario, illustrating the spreading of signals on the 4 antennas. In the following, for the reason of clarity, only the frequency-time grid is illustrated, although the space dimension is present in implementations using multiple antennas.
The coded and modulated symbols are conventionally mapped on the OFDM resource-units in an ordered fashion. The mapping can be done in several different ways. Firstly, the mapping can be done row-wise or column-wise. Secondly, the rows can be filled either from left-to-right or from right-to-left, and thirdly, the columns can either be filled top-to-bottom or bottom-to-top, totally giving eight different ways of mapping. One example is given in FIG. 1a wherein the mapping has been performed row-wise (frequency), left to right and top to bottom.
In combination with spectrum effective transmission technologies such as OFDM. the high speed communication envisaged with present and future communication systems relies on advanced coding schemes. The purpose of the coding schemes is to improve the reliability of the data transfer, and hence reduce the number of retransmissions (lower Bit Error Rate (BER)). The redundancy introduced with the coding generally reduces the transmission rate compared to an ideal loss-less transmission. However, the effective rate is, if an appropriate coding is utilized, improved. The novel coding schemes often referred to as space-time block codes (STBCs). offer redundancy with little effect on the ideal rate. The STBC coding scheme that has received most attention, the Alamouti code, “A Simple Transmit Diversity Technique for Wireless Communications” IEEE Journal on Selected Areas in Communication, vol. 16, no. 8, October 1998, pp. 1451-1458, offers full-rate, i.e. coding redundancy without reducing the rate. According to the coding scheme two symbols are jointly coded and transmitted over two resource-units. i.e. over two sub-carriers or two time slots. The performance depends on the similarity, as regards to channel conditions, between the two resource-units. Other advanced coding schemes have similar requirements
As certain relations between resource-units, for example as identical radio channel conditions as possible, can be a prerequisite for a good performance of the coding, advanced coding schemes such as the Alamouti, inflicts requirements on the mapping of symbols on the OFDM grid.